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#19
Nov1708, 01:45 PM

P: 12

Hi,
as I said, I tried to start it all over, changing the parameters I wasn't so sure last week but I don't see any difference. I'm pretty sure my problem is on the contact parameters definition but I don't know where to go with this. I changed the CAD to keep only the blade I'm interested in. I want to force the roller to follow a specific helix that I have the equation : Rx = R*cos(theta) Ry = R*sin(theta) Rz = R1*theta^9+R2*theta^8+...+R10*theta^0 (or should it be (R1*theta^9+R2*theta^8+...+R10*theta^0)*theta because the Z parameter of an helix = c*theta where c is a constant) It doesn't matter if it rotate on himself or no. In my mind, this path is only for a specific point (because of the Rz equation), this is why I constrained the subdomain with Rx and Ry and "the" point with Rz. To simplify, I'm trying to solve it without friction. The contact pressure is very low; I try to simulate the movement and the contact before augmenting it. Does everything seem to be OK for you? Do I have to set special initial values? 


#20
Nov1808, 02:43 AM

Sci Advisor
PF Gold
P: 1,102

Contact problems and convergence often seem to reside on different planets. If the mesh is "about ok" one of the best ways to accelerate convergence would be to specify an initial contact pressure. Either find something analytical which would match approximately, or then just try to approximate based on the contact problem characteristic dimensions & loading (and improve by iteration for example, or if pressure seems out of the question, a following displacement at the beginning aids as well). Also, trying to simplify the problem if it doesn't converge is one which often works (add constraints, simplify them, then work towards getting to model to work with in the actual analysis case) and pay attention to effects and specification of load stepping.



#21
Nov1808, 11:07 AM

Sci Advisor
PF Gold
P: 2,288

It comes to mind that is the roller is using a frictionless contact condition between it and the rotor, your should constrain the roller rotationally so that it is fully constrained. You would only want to allow the roller to "roll" if the contact condition was frictional.
Contact regions can indeed be very tricky. While I haven't ever actually used COMSOL, I do a lot of analysis in ANSYS and some of my modeling has made me use tricky contact conditions. Some tricks I have used in the past to help the nonlinear problem converge:  Increase the size of the contact region's "pinball region." It's basically the maximum distance between nodes in the contact regions that the solver will try and solve for. Sometimes if an intermediate load step moves a contact region too far into the "target" region, all of the nodes will be outside of the pinball region and prevent convergence since the region will not "work." This can be an especially annoying problem if the contact regions have an initial distance between them that is larger than the pinball region, and the first load step moves the two regions either farther apart or even past each other.  Increase the number of intermediate load steps. If the nonlinear problem doesn't seem to be solving becase you're applying a force in a single step, you can split the solution into intermediate steps to help convergence. Sometimes it helps, but don't know how this is accomplished in COMSOL.  ANSYS has an option where if a contact condition's status changes dramatically between two intermediate load steps, it can automatically split the load step in half to try and enhance convergence. Don't know if COMSOL has this option.  While the solution is in progress (depending on the solver type), it is best practice to see if the force convergence parameter is actually converging. If you're not converging on the first intermediate load step, chances are your load steps are too big or there is something wrong with your model's initial conditions. 


#22
Nov1908, 02:10 PM

P: 12

I was wondering,
is it a better thing to use theta as my parameter (using parametric solver) to simultae the rotation of the roller around the blade or should I include this relation of movement in an equation (with the movement in z for example) and, instead of theta, use the contact pressure as the parameter of the solver. By doing this, I think it will only have more load on the blade (in function of time) so that for example, when the roller will be in the middle of is way frome the beginning to the end of the blade, the pressure will be higher than at the beginning (what is something I don't really want). By the way, I didn't found how to simulate the movement of the roller still (have the equations but can't "see it" or solve it, no, it solves, but there's no movement), I'm just exploring new ways to get an answer... 


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